Bitrot: Arrange for shared keys in SMTP server session tickets.

Otherwise, with clients that enable session tickets, the SMTP session
cache is per-process and largely ineffective.  Older releases should
add SSL_OP_NO_TICKET to SSL options bit mask in the SMTP server only.

The session ticket key validity interval (sum of initial issuing
and retired key validation intervals) must not exceed the SSL session
lifetime.  Otherwise, clients may send valid tickets for expired
sessions, which the OpenSSL server code mishandles (does not send
a replacement ticket, patch pending...).

We set the session lifetime to 2 times the configured cache lifetime
which is also the ticket issuing and retired validation lifetime,
so ticketed sessions last 1 to 2 times the configured session
lifetime and never longer than a session's expiration time.
Documentation rebuild.